The Leyden Jar

The first device capable of storing an electric charge was the Leyden jar. Invented by a German, Ewald G. von Kleist, on November 4, 1745, he made the discovery by accident. While experimenting with electricity, he touched his electric generator to a nail stuck into a medicine bottle through the cork. Later, he received a great shock by touching the nail. Although he did not understand how it worked, he had discovered that the nail and the jar were capable of temporarily storing electrons. Today we would call this device a capacitor. Capacitors are used in every form of electronic equipment.

Von Kleist may have been first, but he is nearly forgotten today. Credit for the Leyden jar is usually attributed to Pieter van Musschenbroek of Leyden, Holland who in 1746 discovered exactly the same thing. Using a jar of water with a metal rod in it, he touched the rod to his electrostatic generator. Nothing seemed to happen, but when the person holding the jar touched the rod, he got a terrific shock. Marketing is everything, and the news of Van Musschenbroek's discovery spread quickly around Europe, and around the world.

The Leyden jar became very important in electrical research. More compact and easier to move than an electrostatic generator, experimenters could charge up their jars and take the stored electricity with them in the laboratory or outside. Benjamin Franklin used Leyden jars in his famous kite flying experiments. Over a century later, Leyden jars and capacitors became important in electric lighting, radio, and many other practical applications.

The Leyden jar is the very same object referred to in the phrase, "catching lightning in a bottle," meaning capturing something powerful and elusive and then being able to hold it and show it to the world.

How it works
The Leyden jar is a cylindrical container made of a dielectric (an insulator, like plastic or glass) with a layer of metal foil on the inside and on the outside. With the outside surface grounded, a charge is given to the inside surface. This gives the outside an equal but opposite charge. When the outside and inside surfaces are connected by a conductor, you get a spark and everything returns to normal.

The amount of charge one of these devices can store is related to the voltage applied to it times its capacitance. In simple terms, capacitance depends on the area of the foil or metal, the type of material between the two layers of foil, and the thickness (generally the thinner the better) of that material.